The fundamental nature of electron transfer between transition metal ion sites in electron-transfer proteins is being studied. Model complexes with electronic and structural characteristics similar to a particular metalloprotein metal site are being prepared and studied to understand what aspects of the electronic structure of the metal site are important in determining the biological functioning of the metalloprotein. Mixed-valence bridged ferrocenes and other molecules are being studied to determine the fundamental nature of electron transfer between metal ions and to detail the nature of the mixed-valence character of metal sites in iron-sulfur proteins. It is of interest to find out whether thermal electron transfer can occur between metal ions that are well separated or is electron tunneling involved. Binuclear complexes are being prepared to study magnetic exchange interactions between metal ions as propagated by extended briding groups. Spin-crossover ferric complexes are under study to elucidate the factors that control the spin-flipping rate in an individual spin-equilibrium molecule and the cooperativity of spin flipping seen for the solid state. Iron-quinone complexes are being characterized.